WASHINGTON — Donald Trump's repeated charges that the race for the White House is being "rigged" against him is unprecedented, historians say. They warn it could undercut the ability of the next president to govern — or even spark violence on Election Day and afterwards.

Never in modern times has a major-party nominee hurled accusations — with no credible evidence — that the election results themselves can't be trusted, a development that could raise questions about legitimacy of the winner. Even in such razor-thin elections as 1960 and 2000, Republican Richard Nixon and Democrat Al Gore accepted results that some of their supporters disputed. Each called on Americans to unite.

"There have certainly been candidates who toyed with apocalyptic language — Teddy Roosevelt running for the GOP nomination in 1912 comes to mind — but none has made questioning the validity of the election a centerpiece of his campaign," says Nicole Hemmer, an assistant professor in presidential studies at the University of Virginia's Miller Center. "There's just no useful historical precedent that I've come across, certainly not at the national level. Historians hesitate to label things unprecedented, but the word is certainly getting a workout this election."

Trailing in new national polls and beset by growing allegations of sexual assault, Trump is lashing out at the news media, the political establishment, a Mexican billionaire and others. "Election is being rigged by the media, in a coordinated effort with the Clinton campaign, by putting stories that never happened into news!" he declared in one tweet Sunday morning.

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On Sunday afternoon, he tweeted: "This election absolutely is being rigged by the dishonest and distorted media pushing Crooked Hillary - but also at many polling places - SAD."

Trump is arguing that the results are being rigged even before Election Day has been held. What's more, as his supporters chant "Lock her up!," he repeatedly has promised that, if elected, he would ask the attorney general to appoint a special prosecutor to investigate Clinton.

Fears of violence

Trump's rhetoric has inflamed some of his followers and raised fears of violence at the polls and after the election.

At a campaign town hall in Newton, Iowa, last week, Trump's running mate, Mike Pence, faced a woman who said she was "ready for a revolution."

"Our lives depend on this election," she said with emotion. "Our kids' futures depend on this election and I will tell you just for me, and I don't want this to happen but I will tell you for me personally if Hillary Clinton gets in, I myself, I'm ready for a revolution because we can't have her in."

When Pence demurred, saying "Don't say that," she went on: "What are we going to do to safeguard our votes? Because we've seen how the Democratic Party is just crooked, crooked, crooked."

Historian Michael Cohen, author of American Maelstrom: The Election of 1968 and the Politics of Division, says he is very concerned about the prospect of political violence. "He's telling his supporters that they didn't lose fair and square, but rather it was stolen," he says.

Hemmer says a "confluence" of disturbing developments are creating a volatile situation. She ticks them off: "Trump's attempt to render the election results illegitimate in advance, the rising temperature at Trump rallies, and Trump's language concerning the media "poisoning" voters' minds."

"If Trump continues to throw gasoline on the fire over the next three weeks, the chances of violence on or after Election Day seem uncomfortably high," she says.

The consequences could resonate well beyond Election Day, Cohen says. "He's undermining the legitimacy of a national election — and without evidence. Assuming Clinton wins, it puts a permanent cloud of illegitimacy around her presidency. How do Republicans in Congress work with her?"

Trump's rhetoric is at odds with the history of American presidential politics, even after the fiercest campaigns and closest returns.

"Even at times when there were serious voting irregularities that would support the claim of a rigged election, as in 1960 or 2000, the losing candidate has respected and reinforced the validity of the process," Hemmer says. "When Richard Nixon lost to John Kennedy, Republican officials pursued challenges to vote counts in Illinois, but Nixon himself publicly distanced himself from those efforts and made clear he accepted Kennedy's victory."

Here's what Nixon said in 1960: "As I look at the board here, while there are still some results still to come in, if the present trend continues, Mr. Kennedy, Sen. Kennedy, will be the next president of the United States. I want, I want Sen. Kennedy to know, and I want all of you to know, that certainly if this trend does continue, and he does become our next president, that he will have my wholehearted support and yours too."

Gore spoke in December 2000 after the Supreme Court ruled 5-4 against a recount of the disputed results in Florida.

"I spoke with George W. Bush and congratulated him on becoming the 43rd president of the United States — and I promised him that I wouldn't call him back this time," Gore said. "I offered to meet with him as soon as possible so that we can start to heal the divisions of the campaign and the contest through which we just passed."

'Poor me!'

Over the weekend, Clinton held no public events as she prepared for the third and final presidential debate on Wednesday. Democratic running mate Tim Kaine responded to Trump's assertions at a Miami rally Saturday night.

"Now that he thinks he’s going to lose, he’s going around and saying, ‘Oh, the whole thing’s rigged. It’s just rigged against me. Poor me!’” the Virginia senator said, calling on supporters to provide such a wide margin of victory that accusations of a stolen election won't stick. "We've got to make sure that the margin that he loses by is so big and so clear and so powerful and so unmistakable, that when he stands up and says, ‘Poor me, it was was rigged against me’ — that nobody will believe him."

“Our democracy relies on confidence in election results, and the speaker is fully confident the states will carry out this election with integrity,” Ryan spokesperson AshLee Strong said in a statement released Saturday. She said Ryan was “fully confident” in the nation’s elections system.

Trump responded on Twitter Sunday afternoon with a dismissive reference to Ryan's nomination as Mitt Romney's running mate in 2012. He called Ryan "a man who doesn't know how to win (including failed run four years ago)."

Meanwhile, more women came forward in news accounts with allegations that Trump in the past had groped and forcibly kissed them against their will. On Saturday, the British newspaper The Guardian quoted Cathy Heller who said the businessman manhandled her in an incident on Mother's Day about 20 years ago during a brunch at Mar-a-Lago, Trump's estate in Florida.

Before the Guardian story, 12 other women came forward last week to describe sexual misconduct by Trump in stories reported by The New York Times, The Washington Post and others. Before the Guardian story, a total of 12 women came forward last week to allege sexual misconduct by Trump.

Trump has flatly denied all the accusations.

As his poll ratings have fallen, Trump's accusations that there is a global conspiracy against him have escalated.

"This election will determine whether we are a free nation or whether we have only the illusion of democracy, but are in fact controlled by a small handful of global special interests rigging the system, and our system is rigged," he told a rally in West Palm Beach last week. "This is reality. You know it, they know it, I know it, and pretty much the whole world knows it. The establishment and their media enablers will control over this nation through means that are very well known. Anyone who challenges their control is deemed a sexist, a racist, a xenophobe, and morally deformed."

Some of his leading supporters chimed in with similar charges Sunday.

On ABC's This Week, former House speaker Newt Gingrich said, "Fourteen million citizens ... picked Donald Trump; 20 TV executives decided to destroy him." On CNN's State of the Union, former New York mayor Rudy Giuliani, a Trump surrogate, charged that election fraud remains a reality in big cities such as Chicago and Philadelphia. "I'm sorry, dead people usually vote for Democrats rather than Republicans," he said.

Ari Fleischer, a former White House press secretary for President George W. Bush, said a decision by Trump not to concede the election would be only "impolite." But he said an argument that the election was somehow stolen would be corrosive, dangerous, and "could really undermine our democracy."

He doubts Trump would go that far, especially if he lost in a landslide. "At the end of the day, I would be surprised if he took that step," he said, "especially if it was a big loss."

"We will absolutely accept the result of the election," Pence said on NBC's Meet the Press. "But the American people are tired of the obvious bias in the national media. That's where the sense of a rigged election goes here." He accused the news media of "an avalanche of continuous negative attacks against my running mate instead of focusing on the real, hard evidence coming out about corruption and pay to play in the Clinton Foundation years."